Biological activity in mammals, including humans, generates electric fields. These fields vary over the body and change depending on physical, emotional, and mental condition. For example, functioning of the brain generates electrical fields that can be detected by monitoring electrical potentials at points on the scalp. The monitored potentials are an example of electroencephalography (EEG) signals. Features of EEG signals such as the frequency spectrum provide an indication of the functioning of the brain. Neurofeedback systems may process EEG signals and generate sounds or other feedback signals. Users may use neurofeedback systems to learn to exercise control over the functioning of their brains.
An example neural feedback system that uses auditory stimulation to provide feedback is the NeurOptimal® system available from Zengar Institute Inc. of Victoria, Canada www.zengar.com. NeurOptimal monitors a user's brainwaves and alerts the central nervous system when it is not functioning smoothly by modulating music that the user is listening to. When brain activity shows signs of turbulence, the music provided by the NeurOptimal® neurofeedback software is momentarily interrupted. This subtle cue alerts the user's brain that it is operating inefficiently. With repeated training sessions, the brain learns to “reset” itself and function more smoothly.
A beating heart also creates a time-varying electric field that can be detected at the skin. The monitored signals are an example of electrocardiography (ECG) signals. One can analyze ECG signals to determine whether the heart beat is normal or abnormal. Similarly, sensors may be placed to measure muscle function using electromyography (EMG).
Patent documents that describe various EEG systems include: U.S. Pat. Nos. 5,230,346; 4,503,860; 4,411,273; 7,729,740; US Publication No. 20110282231; US Publication No. 20110046503; and US Publication No. 20130079659.
Bioelectrical signals including EEG signals are generally small in amplitude. Such signals generally require amplification. As the signals are weak, noise is problematic. The overall performance of a system for monitoring small electrical potentials (for example, EEG, EMG, ECG systems) can be highly dependent on the nature of the sensors used to detect the electrical potentials.
Patent documents that describe sensors which provide amplification include US20130066183, US20120250197, EP1451595B1, and U.S. Pat. No. 8,264,247.
Various types of electrode may be used to detect EEG signals. Some EEG systems use wet-contact electrodes. Wet-contact electrodes are used with conductive gels or pastes to provide low-impedance connections to the user. Wet-contact electrodes can be inconvenient and time consuming to use. Furthermore, conductive gels and pastes are messy and can often bleed between neighboring sensors and cause signal contamination.
Dry electrodes do not use pastes or gels, but rather contact the user directly. The electrical impedance of connections made using dry electrodes is typically much greater than the electrical impedance of connections made using wet contact electrodes. This impedance of connections made with dry sensors can vary significantly due to factors such as skin condition, the presence of hair between the dry electrode and the skin etc.
Non-contact sensors rely on capacitive coupling and do not require an electrically-conductive connection to a subject to operate. There are a number of problems associated with non-contact sensors. The coupling to the field is very weak, making the sensor prone to extraneous noise pickup. Also, typical amplifier designs require finite—although commonly minute—input currents to operate correctly. Non-contact sensors are described, for example, in: Sullivan et al. A Low-Noise, Non-Contact EEG/ECG Sensor IEEE 2007; Cauwenberghs et al. Wireless Non-contact Cardiac and Neural Monitoring. Wireless Health, Oct. 7, 2010. San Diego, USA.; Ross, Recent Patents on Non-Contact Electrodes for Measuring EEG and EKG Recent Patents on Electrical & Electronic Engineering 2013, 6, 2-6; Chi et al., Non-contact Low Power EEG/ECG Electrode for High Density Wearable Biopotential Sensor Networks, Sixth International Workshop on Wearable and Implantable Body Sensor Networks Jun. 3-Jun. 5, 2009. non-contact sensors are also described in U.S. Pat. No. 8,193,821, CA2706956, US20120265080 and U.S. Pat. No. 8,780,512.
US20130039509 and U.S. Pat. No. 827,107 describe headsets equipped with electrodes for detecting bioelectrical signals. EP2709519A1 describes various applications for sensed biosignals. U.S. Pat. No. 7,088,175 describes measuring free space electric fields.
There remains a need for sensors suitable for detecting small electrical potentials that ameliorate some of the disadvantages of existing sensors. In the field of neurosensing and neurofeedback there remains a need for reliable non-contact sensors that can operate in the presence of noise and are suitable for technologies such as electroencephalography (EEG), electromyography (EMG), and electrocardiograms (ECG).